In the field of optical communications, optical fiber collimators are conventionally used to emit signal light, which has been transmitted through an optical fiber, in the form of parallel light from the optical fiber, or in reverse, used to converge parallel light on one end surface of an optical fiber to cause the light to enter the optical fiber. When using such an optical fiber collimeter, interposition of an optically functional element (for example, an optical filter, optical isolator, optical switch, optical modulator, etc.) between a pair of collimator lenses can provide a desired effect on the signal light transmitted through a single mode optical fiber at the light-entering side and then converge the signal light so as to further transmit the light to a single mode optical fiber at the light-receiving side.
Various forms of lenses have been used as collimating lenses for optical fiber collimators. However, cylindrical graded refractive index lenses (also referred to as “rod lenses” or “GRIN lenses”) are generally used because of their ease of polishing and like processing during manufacture, compared to spherical lens and complicated non-spherical lenses. Such graded refractive index lenses work as lenses, for example, converging light, because the interior of the rod glass has a continuous refractive index distribution in the radial direction from the center.
To produce such graded refractive index lenses, ion exchange methods, double crucible methods, CVD methods (vapor-phase deposition methods), sol-gel methods, rod-in-tube methods, and like methods are known as techniques for forming a graded refractive index distribution in the radial direction of a glass rod. Among these, ion exchange is the most typical method for producing a graded refractive index lens, and comprises immersing a homogeneous glass rod in a molten salt containing a monovalent cation (e.g., K+, Tl+, Ag+) to exchange a monovalent cation in the glass (e.g., Na+) for the monovalent cation in the molten salt, thereby forming a graded refractive index distribution. For example, Patent Document 1 discloses a method of producing a graded refractive index lens comprising subjecting a Na-containing glass rod to ion exchange using a molten salt containing Ag+ to form a graded reflective index distribution in the radial direction of the rod.
Further, microlens arrays comprising arrays of lenses with a diameter of about several tens of micrometers to submillimeter, each lens having a graded refractive index distribution formed by subjecting a planer glass to ion exchange, are being used as computer board connectors or light source collimaters.
Optical elements produced by forming such a graded refractive index distribution include optical waveguides. Known methods for forming optical waveguides include thin film deposition methods. Thin film deposition is a method of depositing an optical waveguide layer having silica as a principal component on a substrate made of silicon, etc. More specifically, sputtering methods, CVD methods, flame deposition methods, and the like are known. Disadvantageously, all these methods need high vacuum equipment for the production of waveguides and use complicated production processes, therefore resulting in increased costs. Moreover, in CVD methods and flame deposition methods, in some cases, hazardous gases, such as SiH4, SiCl4 and the like are used, entailing high costs. Furthermore, flame deposition methods have disadvantages in that exposure to the high temperatures of about 1200° C. to about 1300° C. in the production process tends to degrade the substrate, and in addition, causes internal stress in the substrate, increasing the polarization dependence of guided light, as well as other problems.
Other known methods for forming optical waveguides include ion exchange methods. Ion exchange methods use a multi-component glass containing Na+ ions as a substrate and comprise the step of immersing the glass substrate in a molten salt containing K+ ions, Tl+ ions, Ag+ ions or the like to exchange Na+ ions in the glass for K+ ions, Tl+ ions, Ag+ ions or the like in the molten salt. In such an ion exchange method, an electric field may be applied during ion exchange so as to increase the ion exchange rate, ion diffusion rate, etc. Ion exchange methods can increase the refractive index of the portion where ion exchange has been carried out, forming an optical waveguide layer.
Unlike thin film deposition methods, ion exchange methods do not require high vacuum, and the temperature of the molten salt is usually in the range of about 250° C. to about 400° C. Thus the production facilities are low-cost. However, it is necessary to strictly control the composition, temperature, etc. of the molten salt, which influence the rate of ion exchange, the rate of ion diffusion into the glass substrate, etc. Moreover, the temperature at which ion exchange is conducted is influenced by the melting point of the molten salt. Therefore, when producing an optical waveguide having a desired refractive index profile by an ion exchange method using a molten salt, a high level of expertise is needed in the determination of ion exchange conditions such as the composition and temperature of the molten salt, processing time, etc. When performing ion exchange using a molten salt, it is necessary to apply an ion exchange-blocking film to the entire substrate except for the portion where ion exchange is to be performed. Photolithography is generally used to apply an ion exchange-blocking film, but formation of such a blocking film requires a complicated production process. Furthermore, when ions which are prone to oxidation in air are used as the ions to be introduced, ion exchange needs to be performed in a reducing atmosphere.    Patent Document 1: Japanese Unexamined Patent Publication No. 2001-159702